There are two groups of centrifuges for liquid-solid separation: filtering centrifuges and sedimenting centrifuges. Purchas (1981) has given full account of the designs and applications of each type. The major difference between the two is that the former utilizes a perforated bowl through which the fluid (centrate) can pass while the solids are retained inside the bowl. The latter is equipped with a solid (impermeable) bowl, and separation of the fluid is done by forcing it to overflow from the bowl while the solids are retained on its walls. The method of transport of solids and the control of liquid flows can vary widely in both types of centrifuge, and these factors are the ones generally used to subclassify the different machines.
Sedimenting centrifuges remove solids from liquids by causing the particles to migrate radially towards the walls of the centrifuge bowl. The basic types of centrifuges in this category are: a) the high speed, tubular bowl type with manual discharge of solids; b) the skimmer pipe/knife discharge types; c) the disc-type centrifuge; and d) the continuous scroll discharge machines.
A) The tubular Bowl centrifuge generally has a bowl with a diameter of between 15 and 50 cm which rotates at high-speed to generate a settling acceleration of up to about 18,000 g (where g is the acceleration due to gravity) for industrial models and 65,000 g for laboratory models. (This compares to accelerations of <1,000 g for cylindrical solid bowl machines.) The feed slurry jets into the bottom of the bowl through the bowl neck, and a distributor disperses the feed to prevent travel too far along the bowl length. The centrate discharges from the top of the bowl by overflowing into a collecting cover.
B) Some designs use a skimmer pipe and knife to discharge the solids. Bowl diameters can be up to about 1.5 m for larger industrial units, and rotate about the vertical axis to generate up to 1,600 g. The feed enters at the bottom of the bowl and discharges over a lip ring at the top. The solids sediment to the walls of the bowl, where they are allowed to accumulate until the centrate clarity is adversely affected, then the feed is stopped and the solids discharged. When the feed is stopped, the skimmer pipe engages the layer of liquid above the solids and the rotational energy in the liquid causes it to flow up the skimmer pipe and be discharged. The skimmer is lowered into the rotating liquid; but before it contacts the solids layer, the discharging stream is diverted and those solids which are sufficiently fluid are then discharged. If the solids are compacted and will not flow, a knife is used to discharge the solids after the skimmer pipe has been retracted and the bowl slowed to an idling speed of 1 to 2 Hz. After solids discharge the knife is retracted, the bowl accelerated, and the cycle repeated. The cycle is usually fully automated.
C) The disc type centrifuge thickens the suspension to form a thick slurry by generating an acceleration of up to about 12,000 g. The bowl contains between 50 and 150 conical discs, spaced approximately 2 mm apart. The suspension flows towards the axis of the machine through the spaces between the discs; solids settle to the underside of each disc, slide along the disc in the outward direction and from the edge of the disc, are thrown to the wall of the bowl by the centrifugal field. The clarified liquid passes to the center of the bowl and discharges over a weir at the top or bottom. The solids (thickened slurry) are usually discharged through ports at the periphery of the bowl, which open at a timed cycle (opening bowl centrifuge), or continuously through nozzles (nozzle discharge centrifuge) (Figure 1).
D) The continuous scroll discharge (or decanter) centrifuge provides a settling acceleration of up to 4,000 g. Bowl diameters are generally in the range of 10 to 100 cm and are cylindrical with a conical end (a few designs have wholly conical bowls). The feed is introduced through a concentric pipe to an appropriate point along the bowl; the centrifugal force causes the solids to sediment to the wall to leave a clear liquid in the bowl. Inside the bowl, a helical screw conveyor rotates at a speed of up to 2 Hz slower than the bowl and scrolls the sedimented solids to one end of the bowl, up the beach (the conical section), out of the pond (the liquid) and discharges them from the bowl at the end of the conical section. The clarified liquid spills over a weir at the opposite end of the bowl (although in some designs the liquid flows cocurrently with the solids).
The ∑ theory concept is used for the scale-up of sedimenting centrifuges. For 50% capture of particles of diameter D, density ps, suspended in a fluid of density pl and viscosity η, the volumetric flow rate Q is given by:
where u is the settling velocity of the particle in a gravitational field as given by Stokes' Law
∑ has the units of m2. For a given feed and for centrifuges of the same geometry, the ratio Q/∑ is considered constant and permits scale-up calculations.
For the tubular bowl, skimmer pipe and scroll discharge machines, Records (1986) gives ∑ as:
where L is the clarifying length; r1 and r2 are the radii of the liquid surface and the bowl, respectively; and ω is the angular velocity of the bowl. For the disc type machine Alt (1986) gives
where n is the number of discs at an angle of inclination φ, and ri and ro are the inner and outer radii of the discs, respectively. It is important that one uses ∑ or modified ∑ values as recommended by the equipment manufacturer.
Centrifugal filters have developed from simple batch types of machines to sophisticated continuous plant; both batch and continuous equipment are in common usage. Basket, peeler and pusher-peeler centrifuges are examples of batch-operated machines, and conical screen and pusher centrifuges are examples of continuous units. There are many variants within some of these categories of centrifuges.
A) Batch-type centrifugal filters are comparatively simple in design and versatile in application, and represent the first generation of machines (although they are still widely available). The main problem of this type of centrifuge is stabilization of the imbalances resulting from unequal loading—hence the names which have evolved: buffer centrifuges (foundations mounted on rubber buffers), gyroscopic centrifuges (rotating shaft which permits nutation movement), and pendulum centrifuges (rigid mounting in a housing from which the basket is hung). Basket machines can be subdivided into solid and open basket designs; solid basket centrifuges tend to require manual discharge of cake, whereas open baskets allow knife discharge, and either type may be operated from beneath through a base bearing or from overhead via a link suspension or pendulum.
The main advantages of batch centrifuges are high separation efficiency and high purity of the separated products. They are generally operated at variable speeds and are most suitable for washing. A typical cycle is:
acceleration to loading speed;
acceleration to filtration speed (higher than loading speed);
progressive deceleration to low speed and thence to rest; and;
Peeler centrifuges mostly operate at a constant speed to avoid time losses and higher power consumption associated with accelerating and decelerating. The solids are discharged by being peeled from the basket by a knife edge and then dropped into a discharge chute which carries them out of the basket.
The theory of batch centrifuges is not well-developed and is limited to providing an estimate of the instantaneous flow rate of centrate discharging from the machine. The expression for the volume flow rate of centrate is:
where ρ1 is the density of the centrate, ω is the angular velocity of the basket, l is the thickness of the cake which has a specific resistance α, r2 is the inner radius of the basket, r1 is the inside radius of the liquid phase and R is the resistance of the filter medium.
B) Continuous centrifuges constitute the second- and third-generations. Second-generation centrifuges, from which there is continuous discharge of both liquid and solids from the basket, are the pusher and conical screen types. The third-generation centrifuges include the screen decanter, baffle ring, screen baffle and siphon centrifuges.
Conical screen centrifuges are available in a wide variety of designs. A first classification of these is the angle of the screen—wide-angle (slip discharge and guide channel centrifuges) and small-angle screens. Small-angle screen centrifuges are then subdivided according to the method of solids discharge used: vibration (vibration centrifuge); oscillation (oscillating and tumbler centrifuges); metering (worm or scroll screen centrifuges); and pushing (pusher conical screen centrifuge). Conical screen centrifuges may rotate about either the vertical or the horizontal axis.
The pusher centrifuge consists of a rotating perforated drum lined with a slot screen and a push plate reciprocating with a frequency of about 1 Hz, with a variable advance of between 30 and 60 mm. At first sight, the mechanical design of a tumbler appears complicated, but the construction is simple in comparison with other types of continuous centrifuge. This is reflected in low capital and running costs and its applications, which have covered freely filtering materials such as iron ore, coal fines and coarser crystalline materials. Although the shape of the drum is conical like that of a conical basket centrifuge, the tumbler typically enables longer residence times.
The geometry and complex motions of many continuous centrifuges make them extremely difficult to formulate; hence few attempts have been made to do so and adequate design equations do not exist. The motion of particles through pusher centrifuges has been analyzed in detail by Deshun et al. (1991), and through cone centrifuges by Wakeman et al. (1991).
The third-generation represents improvements in process technique being built-in into the continuous centrifuges. In the screen decanter centrifuge, sedimentation and filtration are combined to reduce the total time for separation (and limiting their application to easily filterable products). In the baffle ring centrifuge, the particles bounce against rotating rings to effect a further release of liquid. In the screen baffle centrifuge, the particles bounce against screens. Applications of both the ring and screen baffle machines are limited to granular particles. The siphon centrifuge is an adaptation of peeler and pendulum machines. The siphoning action downstream of the filter medium increases the pressure difference across the basket, leading to an increase in capacity.
Deshun, F., Wakeman, R. J., and Zhong, H, (1991) An Analysis of Gyratory Forces and Wobble Angles in Tumbler Centrifuges, Trans IChemE, 69, Part A, 409-416.
Purchas, D. B. (1981) Solid/Liquid Separation Technology, Uplands Press, London.
Records, F. A. Chapter 6, Hultsch, G. and Wilkesmann, H. (1986) Chapter 12, Solid/Liquid Separation Equipment Scale-up, (Eds. D. B. Purchas and R. J. Wakeman), Uplands Press & Filtration Specialists, London.
Wakeman, R. J. and Deshun, F. (1991) The Control Ring Centrifuge—A New Type of Conical Basket Centrifuge, Trans IChemE, 69, Part A, 403-408.